Ectopic hyperparathyroidism

The patient with a 9 year history of chronic renal failure developed increasing malaise and myalgia. Past medical history included a negative neck exploration for parathyroid adenoma. Biochemical assays revealed corrected serum calcium of 3.2 mmol/l and a parathyroid level of 392 ng/l (normal range 10-50ng/l).

The patient with a 9 year history of chronic renal failure developed increasing malaise and myalgia. Past medical history included a negative neck exploration for parathyroid adenoma. Biochemical assays revealed corrected serum calcium of 3.2 mmol/l and a parathyroid level of 392 ng/l (normal range 10-50ng/l).
Parathyroid imaging was performed using metastable technetium 99 (Tc-99m) pertechnetate subtracted from Sestamibi labelled with metastable technetium 99 (Tc-99m) images, which demonstrated focal increased uptake within the superior mediastinum, to assist in the preoperative localisation of the parathyroid gland (Figure 1).
Thoracic contrast enhanced CT was subsequently performed and revealed an enhancing 2 cm mass within the anterior superior mediastinum (Figure 2).

Patients with renal failure often develop secondary hyperparathyroidism in order to attempt control of their hypocalcaemia. Occasionally tertiary hyperparathyroidism develops in which the parathyroid effectively becomes a self-regulating parathyroid adenoma with elevated serum calcium and parathyroid hormone levels.
Diagnosis of hyperparathyroidism is essentially by biochemical assay of corrected serum calcium levels and parathyroid levels. Preoperative imaging localisation is beneficial in enabling a miniparathyroidectomy to be performed; which has been show to have improved cosmetic results, and a reduced length of hospital admission. Preoperative localisation is often with ultrasound or radioisotope imaging either alone or in combination.
Subtraction imaging with the Sestamibi / pertechnetate technique is effective with both sensitivity and accuracy of 90-95% for the detection of parathyroid adenomas. False negative results with the Sestamibi technique is often due to poor uptake of Sestamibi by clear cell adenomas, which have a reduced oxyphil cell content and have a prevalence of in 5 -10%. Hence ultrasound in these instances can reduce the overall imaging false negative rate.
Radiologically ectopic parathyroid adenomas can be difficult to localise. With CT, a meticulous technique is advised with thin slices at least 5mm performed following intra-venous contrast. The small parathyroid adenoma can be difficult to distinguish from normal sized lymph nodes.
MRI has advantages since on STIR or fat suppressed T2 weighted sequences from the skull base through the mediastinum it should demonstrate parathyroid adenomas as hyperintense foci. Correlation of CT or MRI with sestamibi SPECT images improves anatomical localisation and hence surgical planning.
Hyperparathyroidism often has radiographic manifestations such as renal stones or pancreatic calcification on the abdominal radiograph, osteopenia with subperiosteal resorption of the radial aspects of the index and middle finger’s phalanges, and often vascular calcification. Ectopic parathyroid glands are quite common with an incidence of between 1 to 5%. In the embryo the inferior glands start in the third (higher) pharyngeal pouch and the superior glands in the fourth (lower) pharyngeal pouch crossing during development. Most individuals have 4 glands although there reports of between 2 to 8 parathyroid glands. The radiological differential diagnosis for an anterior mediastinal mass of this size in addition to a parathyroid adenoma would include thymoma, teratoma, intrathoracic thyroid gland and lymphadenopathy due to lymphoma, metastasis or granulomatous disease. Our case illustrates the benefit of routinely acquiring fields of view of the neck and the thorax during Sestamibi imaging of the parathyroid glands.